While there may or may not be any benefit in being homosexual, either to the individual or society, the important point is that if you don't reproduce, you don't pass on your genes. No matter how long you live, how much fun you have, or how you benefit society, if you don't reproduce, your genes die with you.

A gene can be said to have been successful if it perpetuates itself, even at the expense of the individual that it codes for. Which is why all kinds of genetic illnesses appear in old age - an individual susceptable to osteoporosis after the age of 50 is equally likely to reproduce as an individual who doesn't. In modern society, even genetic flaws that would prove fatal at a pre-reproductive age in lower animals, such as blindness, deafness, diabetes, or even hemophilia do not result in a significant reproductive disadvantage. These conditions can be inheirited, as the parents were able to reproduce, despite the 'defective' genes.

A genetic predisposition to homosexuality over heterosexuality, however, would significantly reduce the chance of sexual intercourse with an individual of the opposite sex, and therefore the chance of the individual reproducing, and the 'gay gene' being passed on.

The only possibilities for the survival of a 'gay gene' are for it to continually appear as a spontaneous mutation, for it to be recessive, either imparting a small reproductive advantage (or at least imparting no reproductive disadvantage) when paired with a dominant 'straight gene'2, or for it to be expressed by a combination of very many genes, the way eye, hair or skin colour is3.

1 - Barring diseases that affect reproduction directly, such as Down's syndrome

2 - An example of this is the genes for warfarin resistance in rats - ww rats bleed to death after exposure to warfarin, as their blood does not clot. Ww rats are not significantly affected by warfarin, and do not have any crippling deformities. WW rats are not significantly affected by warfarin either, but need substantially more vitamin K in their diet if they are to survive.

3 - Skin colour is expressed by twomany4 pairs of genes, the effects of which add together to make the skin of an individual one of fivelots of4 different colours. Due to the interations of these pairs, it is possible for two individuals with the same skin colour to have offspring with a different one, the exception being parents who are either both pairs fully recessive (white), both pairs fully dominiant (black), or one pair fully dominant, one pair fully recessive.

4 - Nothing on the internet seems to agree what the number is. If you know, please tell me :)